The present invention relates generally to thermal ink jet printers, and more particularly to the supplying of power signals to the ink ejection elements of thermal ink jet printers.
Inkjet hardcopy devices, and thermal inkjet hardcopy devices such as printers, plotters, facsimile machines, copiers, and all-in-one devices which incorporate one or more of these functions in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devicesxe2x80x9d, Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal[Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted from a printhead onto the print media such as paper or transparency film during a printing operation, in response to commands electronically transmitted to the printhead. These drops of ink combine on the print media to form the text and images perceived by the human eye. Color inkjet printers use a number of different ink colors in order to form a wide range of colors and intensities. The colors can be produced through the use of dye or pigments in the ink. Printheads for one or more color inks may be contained in a print cartridge. The ink supply for the printheads may be contained in the print cartridge housing the printhead, or ink may be continuously or intermittently supplied to the printhead from an ink supply located elsewhere. An inkjet printer frequently can accommodate two to four print cartridges, or more. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the media during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row and column format on the media which is to be printed. Each print cartridge typically has an arrangement of individually controllable printhead ink ejection elements for controllably ejecting the ink onto the print media, and thus a certain width of the media corresponding to the layout of the ink ejection elements on the print cartridge, can be printed during each scan, forming a printed swath. The printer also has a print medium advance mechanism which moves the media relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the media with the advance of the media relative to the printheads, ink can be deposited on the entire printable area of the media.
Each ink ejection element, or firing unit, includes an ink chamber connected to an ink source, and to an ink outlet nozzle. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet printers, the transducers are thin film firing resistors that generate sufficient heat during application of a brief voltage pulse to vaporize a quantity of ink sufficient to expel a liquid droplet.
A power source in the printer connects to the print cartridge to supply electrical power (a certain amount of current at a certain voltage) to the firing resistors in the ink ejection elements for a certain amount of time in order to provide the electrical energy required to eject ink drops from the elements. The energy applied to a firing resistor affects its performance, durability, and efficiency. It is well known that the firing energy must be above a certain threshold, known as the turn-on energy, to cause a vapor bubble large enough to expel a drop to nucleate. Above this threshold is a transitional range, in which increasing the energy increases the drop volume expelled. Above a higher threshold at the upper limit of the transitional range, drop volumes do not increase with increasing energy. It is in this upper range in which drop volumes are stable even with moderate energy variations that printing optimally takes place, because the variations in drop volume that cause disuniformities in printed output can be avoided when operating in the upper range. If the applied energy levels increase above this optimal zone, however, drop volume uniformity is not compromised, but rather energy is wasted resulting in excessive temperature rise, and the printer components are prematurely aged due to excessive heating and ink residue build up.
For achieving high print quality, it is frequently desirable to print using different drop volumes for different color inks, or for different shades of the same color ink. Therefore, the design of the printheads may differ from ink to ink in order to produce different stable drop volumes. Different stable drop volumes require different amounts of firing energy; the amount of firing energy required to produce stable drop volumes is generally proportional to the stable drop volume. For example, an ink ejection element designed to produce 30 picoliter drops requires approximately 1.5 times the firing energy required to produce 20 picoliter drops from a differently-designed ink ejection element.
Producing different firing energies for different printheads with different stable drop volumes can be problematic if the printer uses a power supply having only a single output voltage. If a firing pulse of the same voltage is applied to the ink ejection elements of all printheads, then the firing time (the amount of time that the voltage is applied to the ink ejection elements) must be varied in order to provide the proper optimal energy. While this solution is adequate if the required .amount of variation in firing times is not too great between different printheads, there are limitations as to how much the firing time can be varied to compensate for different printheads which, if they are designed to deliver different drop volumes, require different firing voltages. If the voltage applied to a particular printhead is too low for that printhead, requiring that the pulse be lengthened in order to provide the proper firing energy, the pulse may become so long as to undesireably reduce the frequency at which the ink ejection elements can be fired, slowing down printing from the printer. Conversely, if the applied voltage is too high for the printhead, requiring the pulse be shortened in order to provide the proper firing energy, the voltage may be so high and the pulse so short as to cause premature aging and possible failure of the printhead. Thus the voltage appropriate to the design of each particular print cartridge must be supplied in order to avoid these problems, resulting in a need for multiple power sources in printing systems which use printheads having different drop volumes.
The need for supplying different power supply voltages to different print cartridges can also arise even in printheads having the same stable drop volume and firing energy. Parasitic electrical resistances within each print cartridge have the effect of reducing the firing voltage applied to the ink ejection elements below the voltage which is supplied to the electrical interconnection pads of the print cartridge by the power supply in the printer. Manufacturing tolerances can result in the parasitic resistances varying from print cartridge to print cartridge. Since the power supply voltage can only be set to match the parasitic resistances of one of the print cartridges, other print cartridges having different parasitic resistances must operate with non-optimal voltages and thus can suffer from the slower printing or premature aging problems discussed above. To provide the highest print quality would require either that print cartridges be xe2x80x9cmatchedxe2x80x9d to each other, which is impractical in a printing system with individually replaceable print cartridges, or that manufacturing tolerances be set more tightly, which would likely result in increased print cartridge cost.
Even within an individual printhead, a similar need for supplying differing input voltages can arise. In highly multiplexed printheads having a large number of ink ejection elements, different sets or groups of the elements may each be powered by a different common voltage line. If the parasitic resistances in the current path from the electrical interconnection pads to the firing resistors is different for different groups of the ink ejection elements in the printhead, different voltage levels will be required to be supplied to the interconnection pads in order to compensate for the voltage drop through the parasitic resistances and thus provide the required firing energy to each ink ejection element group. In addition to differing parasitic resistances in the current paths of different ink ejection element groups, the number of ink ejection elements that are simultaneously fired in a group can affect the firing energy. When a larger (or smaller) number of resistors is fired in one group compared to another, the increased (or decreased) current drawn from the power supply causes a larger (or smaller) voltage drop through the parasitic resistances, which in turn requires a higher (or lower) output voltage from the power supply applied to the interconnection pads in order to provide the required firing energy to the elements.
In the past, thermal inkjet printers have supplied only a single firing voltage to an individual print cartridge. In addition, past thermal inkjet printers have supplied the same firing voltage to the print cartridges and printheads for different colored (i.e. non-black) inks, with either the same or a different firing voltage supplied to the black ink print cartridge. Supplying inappropriate firing voltages to thermal inkjet printheads can result in less than optimal print quality or a reduction in printhead life. Accordingly, there is still a need for a multicolor thermal inkjet printer which provides appropriate firing voltages to different groups of ink ejection elements in order to provide printed output of high quality.
In a preferred embodiment, the present invention provides an inkjet printing system with a plurality of power sources, each of which can be connected to a group of ink ejection elements and can be independently set to a different voltage level, in order to provide the appropriate firing voltages to each of the different ink ejection element groups. The system has a carriage movably mounted in the frame with respect to the media. The carriage has at least one slot into which a print cartridge containing groups of ink ejection elements can be removably mounted. Each of the ink ejection elements is individually actuable by the application of a firing energy to emit ink drops of a given drop volume. In one preferred embodiment, each ink ejection element group emits drops of the same color ink, while in an alternate preferred embodiment certain different groups each emits drops of a different color ink. In some embodiments, at least two of the groups emit drops of different color inks, each of which have the same drop volume. The ink ejection element groups can be disposed in print cartridge in a number of alternative arrangements. One or more of the ink ejection element groups are disposed in each print cartridge. In some embodiments, all groups are disposed in a single print cartridge. In other embodiments, all groups for a single color ink are located in the same print cartridge. In yet other embodiments, print cartridges contains ink ejection element groups for only a single color ink, or for multiple different color inks. The plurality of power sources preferentially includes a plurality of power supplies, each of which has an independently settable output voltage. Typically, each of the power supplies has an input line connected to the AC power mains. In some embodiments, at least two of the plurality of power supplies have output ground lines which are commonly connected. In an alternate embodiment, the plurality of power sources includes at least one power supply having an output connected to a plurality of voltage regulators, each of which provides independently settable output voltages. Typically, at least two of the voltage regulators have output ground lines which are commonly connected.
In an alternate embodiment, the present invention provides a print cartridge for printing drops of different inks using an inkjet printer. The cartridge includes a printhead die having multiple groups of ink ejection elements. The printhead die also has multiple firing pulse inputs, certain of which are commonly connected to certain ink ejection element groups for controllably ejecting drops of ink. The cartridge also includes a conductive circuit connected to the printhead die, the circuit having multiple power interconnect pads, each of which receives a firing pulse, and electrically conductive traces for conducting the firing pulses from the interconnect pads to the firing pulse inputs of the printhead die. Different color inks may be emitted by different ink ejection element groups; the colors preferentially include black, magenta, cyan, yellow, light magenta, light cyan, dark magenta, and dark cyan color inks. The drop volume of each emitted ink drop may be different in some embodiments for different ink ejection element groups, or for ink ejection groups associated with different color inks. Alternatively, the drop volumes may be substantially the same for at least some of the ink ejection element groups for differently colored inks. Each of the firing pulses has a voltage provided by a power source electrically connected to the power interconnect pads. In some embodiments, the voltages are different for some of the firing pulses. These different voltages are preferably provided by multiple independent power supplies which are electrically connected to different power interconnect pads. In embodiments in which each ink ejection element group has a parasitic resistance, the different voltage values compensate for the different parasitic resistances so as to provide a predetermined firing energy to the corresponding ink ejection element group. In other embodiments in which each ink ejection element group has a predetermined drop volume, the different voltage values provide a corresponding predetermined firing energy to each of the groups.
The present invention may also be implemented as a method for printing with an inkjet printer. The method includes providing multiple ink ejection element groups which each selectively eject ink in response to one of the firing pulses, and providing independently settable firing voltage levels for each of the firing pulses. Power sources may be electrically connected to the groups to provide the firing pulses. The voltage levels are set to an appropriate voltage value for the group, and the firing pulses are selectively generated as governed by the data to be printed so as to emit ink drops. The ink ejection elements from which ink drops are to be printed are selected based on the print data. The voltage value for each group may be set so as to emit a desired ink drop volume from the ink ejection elements. In addition, parasitic resistances of the ink ejection elements in a group may be determined, and the voltage value for the group set so as to compensate for the effect of the parasitic resistances. The ink ejection elements may be housed in one or more print cartridges, with a supply of ink provided to each ink ejection element group in the cartridge. The supply of ink may alternatively be located in the print cartridge, in a reservoir detachably connected to the print cartridge, and in an off-carriage reservoir fluidly connected to the print cartridge.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.